The subject of the invention is a method and a device for controlling the temperature of the exhaust gases of a combustion engine, particularly during phases of regenerating the post-treatment system.
In order to reduce the pollution produced by internal combustion engines it is necessary to fit increasingly complex gas post-treatment systems in the exhaust line, particularly in the case of lean-burn engines.
Unlike a conventional oxidation catalytic converter which eliminates carbon monoxide and hydrocarbons still to be burnt, such systems are designed notably to reduce the emissions of particulates and of nitrogen oxide and operate discontinuously or alternately.
What happens in such systems is that the pollutants are trapped and stored in normal engine operation and are periodically treated, during regeneration phases, which require a special combustion mode in order to guarantee the thermal and/or fuel richness levels needed.
To do this, the rate of fuel injection into the combustion chambers of the cylinders can be set as required in order to operate in a richer or less-rich combustion mode and this results in a variation in the temperature of the exhaust gases and in the proportion of oxidizing and reducing agents in these gases.
Furthermore, in the case of a vehicle engine, the flow rate and temperature of the exhaust gases are also, at each instant, dependent on the engine speed demanded by the driver to suit the driving conditions.
Operation of a post-treatment system has therefore to take various factors into consideration.
For example, in order to reduce emissions of nitrogen oxides NOx in an overall oxidizing mixture such as the exhaust gases of an engine, a catalytic converter comprising a means of accumulating the nitrogen oxides and known as a “NOx-trap” is usually arranged in the cleaning system and is built into the exhaust line and traps the nitrogen oxides emitted during normal engine operation. The operation of a NOx-trap catalytic system such as this is described in detail, for example, in document EP-A-0 580 389.
A catalytic converter such as this is periodically regenerated by running the engine rich for a certain length of time in order to break down the nitrates by releasing NOx which is then reduced to nitrogen by the reducing agents such as H2, HC and CO contained in the exhaust gases, in the way described in the abovementioned document.
Likewise, in order to eliminate particles of soot present in the exhaust gases, the post-treatment system normally comprises a particulate filter, for example a catalytic filter, the interior wall of which is covered with a coat of a material impregnated with precious metals, known as a “wash coat” that performs an oxidation function aimed at reducing the temperature at which the particles of soot are burnt.
In order to prevent a particulate filter such as this from becoming blocked with the soot it is necessary periodically to perform a regeneration which consists of burning the soot by raising the temperature of the exhaust gases in the particulate filter to a setpoint temperature of the order of 600° C. This rise in temperature can be obtained with a degradation in engine efficiency, by appropriate regeneration-aid means.
To this end, it is also possible to position an additional injector in the exhaust, upstream of the catalytic converter, in order to vary the proportions of oxidizing and reducing agents.
Further, to optimize the treatment of all the pollutants, it is necessary to have tight control over the storage and regeneration phases of the post-treatment members by regulating, as far as possible, the thermal power developed within these traps in order to optimize the combustion of soot in the case of the particulate filter and the filling, the reduction of nitrogen oxides and the desorption of sulfur in the case of a NOx-trap, because these combustion, oxidation, adsorption or reduction reactions are directly dependent on the temperature of the support of these traps and of the gases passing through them.
However, it is also necessary to avoid any potential temperature spikes that might carry the risk of damaging the catalytic converter, by controlling the thermal power leaving the first post-treatment system which is generally of the oxidation catalytic converter or NOx-trap type, so as to keep the temperature within a window close to the maximum temperature of use.
In a known way, such control may be had by measuring the catalytic converter internal temperature using sensors placed within this converter. However, installing a sensor within the catalytic converter is difficult and presents risks to the integrity of the monolith of which the catalytic converter consists. It is therefore preferable to place the temperature sensor downstream of the catalytic converter, thermal control at the exit from the first post-treatment system being afforded in a “state feedback” control system from a measurement of the temperature of the gases leaving this system. However, the response of such a system is slow and very delayed and, what is more, highly sensitive to changes in inertia of the monolith caused by variations in the temperature of the gases entering the post-treatment system and/or variations in the flow rate of these gases. As a result, the temporal characteristics of the response of such a method at the control level limit the performance of state feedback control in terms of adhering to setpoint values and it may be impossible to avoid occasionally exceeding the maximum permissible temperature. In addition, because the method is so sensitive to the changes in operating conditions, it will be very difficult to achieve adequately stable control over the entire engine operating range without determining its parameters for each of the operating conditions, something which would be inadmissible in terms of the amount of memory that would have to be devoted to this function.